Devil King Sadao is only one step away from conquering the world when he is beaten by Hero Emilia and forced to drift to the other world: modern-day Tokyo. As “conquering the world” are the only skills the Devil King possesses—and are obviously unnecessary in his new situation—he must work as a freeter to pay for his living expenses! MyAnimeList

This Demon Lord was actually extremely evil in his own world. His true form can be viewed as Satan himself. The flashback did show him trying to conquer the world with magical power but failed against Emily.